A fullerene is a form of pure carbon that is arranged in a three dimensional cage-like structure. These structures are generally spherical or oblong with a central cavity. They are molecules having the formula C.sub.20+2m, wherein m is an integer. Use of the term "fullerene" herein refers to any fullerene or fullerene derivative, including metal encapsulating fullerene derivatives or metallic fullerene endohedral complexes, metallic fullerene exohedral complexes, and substituted fullerene derivatives or fulleroids.
The discovery of Buckminsterfullerene, a C.sub.60 spherical allotrope of carbon, as disclosed in Kroto, H. W., Heath, J. R., O'Brien, S. C., Carl, R. F., Smalley, R. E.; "C.sub.60 : Buckminsterfullerene"; 318 Nature, pp. 162-163 (November 1985), precipitated a flurry of activity directed to understanding the nature and properties of fullerenes. Since their discovery, they have been evaluated for their use as lubricants, semiconductors and superconductors.
Fullerenes have been synthesized by ablating graphite with a laser, by burning graphite in a furnace, and by producing an arc across two graphite electrodes in an inert atmosphere. Other techniques applied to synthesize fullerenes include using negative ion/desorption chemical ionization techniques and a benzene flame. The most common method for synthesis today is the Huffman-Kratschmer carbon arc method. It consists of heating pure carbon in the form of graphite to plasma temperatures by using graphite electrodes in an inert atmosphere (usually helium). This leads to the creation of a soot, from which the fullerenes may be separated. Approximately 10 to 15 percent of the soot contains soluble fullerenes.
The separated crude fullerene mixture consists of 65 to 85 percent C.sub.60 and 10 to 30 percent C.sub.70, with higher fullerenes making up the balance of material. Such higher fullerenes include all fullerenes greater than C.sub.70 and generally include the C.sub.72 -C.sub.200 as the soluble fullerenes.
Due to their highly similar structure, solubility, and reactivity, the only characteristic that significantly differentiates the various fullerenes is their molecular weight. Such similarities have made separation of the discrete fullerene components difficult. Many methods have been tried in an attempt to separate fullerenes but, until recently, few methods were successful.
Most known separation methods employ a column chromatography technique. Column chromatography uses a tube that is open at the top with a valve at the bottom to control the flow of liquid through the tube. The column is filled with a substance that has some affinity for the material that one is trying to separate (stationary phase). Prior to entering the column, the material to be separated is usually dissolved. After placing the material to be separated into the column, a solvent is poured through the column. The material to be separated has some affinity to the column's stationary phase and some affinity to the solvent, so it moves through the tube slower than the solvent. The various molecules one is trying to separate will have differing amounts of affinity to the stationary phase and a "banded" separated product will result.
Most attempts to separate fullerenes have required the use of large amounts of stationary phase and solvent and an inordinate amount of time to separate. The use of standard silica gel alone as the chromatography stationary phase does not work effectively and neutral alumina requires prohibitively large solvent volumes.
Fullerenes also have low solubilities in many common solvents, like hexane. Their low solubility in inexpensive, common solvents, contributes to the ineffectiveness of performing separations on common stationary phases such as silica gel. Continued investigation of the potential utility of these materials is dependent on developing separation methods which facilitate isolation of gram quantities of fullerenes.
Recently, it was discovered that the use of an activated charcoal and silica gel mixture can provide adequate and cost effective separation of gram quantities of C.sub.60 and C.sub.70. As disclosed in U.S. Pat. No. 5,310,532, which is incorporated herein in its entirety by reference, Tour et al. found that activated charcoal columns containing Norit-A and silica gel efficiently separated C.sub.60. Further development led to a method for isolation of C.sub.70 by modifying the mobile phase. This process is disclosed in U.S. patent application Ser. No. 08/238,640 (filed on May 5, 1994) to Tour et al.), which is incorporated herein in its entirety by reference. These two developments have enabled the production of gram quantities of C.sub.60 and C.sub.70 fullerenes. Even with these improved techniques, however, the separation of gram quantities of C.sub.70 takes about 20 hours and a chromatography column one meter long. In addition, neither of these methods allows efficient isolation of gram quantities of the higher fullerenes (&gt;C.sub.70). Although the procedures disclosed in the '532 patent and the '640 patent application work well for obtaining C.sub.60 and C.sub.70 fullerenes, the processes do not sufficiently separate the fullerenes greater than C.sub.70. These methods result in a coeluted mixture of the higher fullerenes.
A GPC method of separating fullerenes has been tried previously. In particular, standard gel permeation columns utilizing nonfunctionalized polystyrene as the stationary phase have been investigated. In such procedures, the stationary phase nonfunctionalized polystyrene has been highly crosslinked and highly porous. The amount of crosslinker utilized has usually been on the order of 10 to 20 percent by weight of the monomer. Such separation techniques have been unsuccessful in obtaining substantial amounts of separated fullerenes. These techniques have been completely unsuccessful in separating the higher order fullerenes above C.sub.70.
The desirability of obtaining purified fullerenes is indicated by typical prices on the market. Using today's separation techniques, C.sub.70 fullerenes can cost $325/gram (98% purity), C.sub.76 can cost $40/milligram (95% pure), and C.sub.84 can cost $30/milligram (95% pure). Typically the higher fullerenes are sold as a mixture of C.sub.84 and above fullerenes. Such unseparated mixtures can typically cost $15/milligram.
Higher fullerenes are currently separated either by high pressure liquid chromatography (HPLC) employing the Buckyclutcher I stationary phase, or by gel permeation chromatography (GPC). These methods of purification, however, have limitations. For example, they typically (1) have limited load capacities, (2) exhibit poor resolution of the higher fullerenes, (3) require multiple (as many as 30) passes through the column to obtain even sub-milligram amounts, and (4) require expensive specialty columns for efficient separation. Additionally, in order to maintain adequate separation profiles on most high pressure liquid chromatography-based stationary phases, potent fullerene solvents cannot be used, thereby inhibiting the dissolution of the higher fullerenes.
These current techniques simply do not provide adequate higher fullerene separations on a preparative scale. It would be desirable to develop procedures that could be used to isolate macroscopic quantities of the higher fullerenes.